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Metal Matrix Feedstock for Additive Manufacturing

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: Develop a metal matrix composite (MMC) feedstock that can be used in additive manufacturing to produce metal matrix composite parts. The desired feedstock would be analogous to the slit tape used in the polymer composites industry for tape placement. This MMC tape can then be used in ultrasonic additive manufacturing (UAM), or similar process, to create MMC components. The feedstock should have greater than 50% fiber volume and have no fibers present on the surface of the tape. 

DESCRIPTION: The Army has a need for strong lightweight structures across multiple Cross Functional Teams (CFTs). Not only in primary structures such as gun tubes, muzzle brakes, and air frames but also in other components such as projectile bodies. Polymer composites are often called on for these applications but they lack high temperature capability and are often weak in the matrix dominated direction. Metal Matrix Composites (MMC) offer the possibility to obtain steel like strength and stiffness in the fiber direction with the density of aluminum. At the same time, the MMC retains aluminum level strength and stiffness in the matrix dominated direction. The problem with MMC's has always been fabricating them. Generally this has been done by either consolidating powder or infiltrating molten aluminum into a ceramic fiber architecture. Both of these tend to be expensive processes and severely limit the size of the part that can be fabricated. What is needed is a feedstock / process that is analogous to the fiber / tape placement process used in polymer composites. In that process a fully consolidated tape of either thermoset or thermoplastic material is used to build a composite structure one layer at a time via sheet lamination. For thermosets the part is cured after this process. For thermoplastics the part is fully consolidated during the process. There have been attempts to use Ultrasonic Additive Manufacturing to create MMC components out of sheets of MMC material. These efforts have met with mixed results. To date the MMC feedstock has fibers on its surface which are broken during the UAM process and damage the sonotrode. The work around for this was to use a thin sheet of pure aluminum between the MMC feedstock and the sonotrode but that severely lowered the possible fiber volume fractions. This topic seeks to develop an MMC feedstock that can be used via UAM (or similar solid state joining process) to fabricate MMC parts of arbitrary size while maintaining a fiber volume fraction greater than 50%. The feedstock and manufacturing process should retain the Additive Manufacturing capabilities of UAM in that interior voids and features are capable of being produced. 

PHASE I: Develop a process to fabricate metal matrix composite (MMC) feedstock with a fiber volume greater than 50%. The feedstock should be in tape form with a thickness on the order of 0.005" to 0.015" thick and a width on the order of 0.25" to 0.5". The preferred composite composition is an aluminum matrix with continuous aluminum oxide fibers (Nextel 610 or equivalent). The fibers should be uniformly dispersed throughout the tape but the surfaces should be pure matrix material. ASTM tests should conducted to demonstrate good adhesion between the fibers and matrix, and to determine the mechanical properties of the feedstock. These properties should be compared to theoretical predictions for the same fiber loading. Several layers of the MMC feedstock shall be consolidated via an additive manufacturing process and assessed for layer adhesion, fiber volume, fiber distribution and mechanical properties. The deliverable shall be 5 lbs of the MMC feedstock. 

PHASE II: Refine the feedstock and produce it using a process representative of plant-scale production manufacturing. Increase the fiber volume with a threshold of 55% and a goal of 65%. No fibers should be visible on the tape surface. Sample MMC parts will be made and tested for mechanical properties. Minimum set of properties that shall be tested for are: longitudinal strength and modulus, transverse strength and modulus, Poisson's ratio, shear strength and modulus, compressive strength and modulus, and fiber volume. Minimum levels for longitudinal stiffness and strength are 30 Msi (207 GPa) and 210ksi (1450 MPa) respectively. All tests will be conducted according to relevant ASTM standards. Tests should be done to adhere the MMC feedstock to a steel substrate via additive manufacturing methods. The material deliverable will be a mutually agreed upon MMC part with a volume exceeding one cubic foot and with internal features. Additionally 25 lbs of feedstock shall provided. 

PHASE III: In collaboration with the prime contractor and Benet Labs, apply a wrap to a complete gun tube for live fire testing in an operational environment. Explore automotive, down-well piping, and manufacturing technology applications for the material. Adapt the low-cost manufacturing process to material applications with less stringent temperature requirements. 

REFERENCES: 

1. L. Burton, R. Carter, V. Champagne, R. Emerson, M.l Audino, and E. Troiano, Army Targets Age Old Problems with New Gun Barrel Materials, AMPTIAC Quarterly, v8n4, 2004. http://ammtiac.alionscience.com/pdf/AMPQ8_4ART08.pdf

KEYWORDS: Advanced Composites, Additive Manufacturing, High Temperature Composites, Metal Matrix Composites 

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